Commit b495021a authored May 22, 2025 by Jiri Slaby (SUSE) Committed by Greg Kroah-Hartman May 22, 2025

tty: serial: 8250_omap: fix TX with DMA for am33xx



Commit 1788cf6a ("tty: serial: switch from circ_buf to kfifo")
introduced an error in the TX DMA handling for 8250_omap.

When the OMAP_DMA_TX_KICK flag is set, the "skip_byte" is pulled from
the kfifo and emitted directly in order to start the DMA. While the
kfifo is updated, dma->tx_size is not decreased. This leads to
uart_xmit_advance() called in omap_8250_dma_tx_complete() advancing the
kfifo by one too much.

In practice, transmitting N bytes has been seen to result in the last
N-1 bytes being sent repeatedly.

This change fixes the problem by moving all of the dma setup after the
OMAP_DMA_TX_KICK handling and using kfifo_len() instead of the DMA size
for the 4-byte cutoff check. This slightly changes the behaviour at
buffer wraparound, but it still transmits the correct bytes somehow.

Now, the "skip_byte" would no longer be accounted to the stats. As
previously, dma->tx_size included also this skip byte, up->icount.tx was
updated by aforementioned uart_xmit_advance() in
omap_8250_dma_tx_complete(). Fix this by using the uart_fifo_out()
helper instead of bare kfifo_get().

Based on patch by Mans Rullgard <mans@mansr.com>

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Fixes: 1788cf6a ("tty: serial: switch from circ_buf to kfifo")
Link: https://lore.kernel.org/all/20250506150748.3162-1-mans@mansr.com/


Reported-by: Mans Rullgard <mans@mansr.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20250522053835.3495975-1-jirislaby@kernel.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

parent 81cf4d7d

drivers/tty/serial/8250/8250_omap.c

+10 −15

Original line number	Diff line number	Diff line
		@@ -1173,16 +1173,6 @@ static int omap_8250_tx_dma(struct uart_8250_port *p)
		return 0;
		}

		sg_init_table(&sg, 1);
		ret = kfifo_dma_out_prepare_mapped(&tport->xmit_fifo, &sg, 1,
		UART_XMIT_SIZE, dma->tx_addr);
		if (ret != 1) {
		serial8250_clear_THRI(p);
		return 0;
		}

		dma->tx_size = sg_dma_len(&sg);

		if (priv->habit & OMAP_DMA_TX_KICK) {
		unsigned char c;
		u8 tx_lvl;
		@@ -1207,18 +1197,22 @@ static int omap_8250_tx_dma(struct uart_8250_port *p)
		ret = -EBUSY;
		goto err;
		}
		if (dma->tx_size < 4) {
		if (kfifo_len(&tport->xmit_fifo) < 4) {
		ret = -EINVAL;
		goto err;
		}
		if (!kfifo_get(&tport->xmit_fifo, &c)) {
		if (!uart_fifo_out(&p->port, &c, 1)) {
		ret = -EINVAL;
		goto err;
		}
		skip_byte = c;
		/* now we need to recompute due to kfifo_get */
		kfifo_dma_out_prepare_mapped(&tport->xmit_fifo, &sg, 1,
		UART_XMIT_SIZE, dma->tx_addr);
		}

		sg_init_table(&sg, 1);
		ret = kfifo_dma_out_prepare_mapped(&tport->xmit_fifo, &sg, 1, UART_XMIT_SIZE, dma->tx_addr);
		if (ret != 1) {
		ret = -EINVAL;
		goto err;
		}

		desc = dmaengine_prep_slave_sg(dma->txchan, &sg, 1, DMA_MEM_TO_DEV,
		@@ -1228,6 +1222,7 @@ static int omap_8250_tx_dma(struct uart_8250_port *p)
		goto err;
		}

		dma->tx_size = sg_dma_len(&sg);
		dma->tx_running = 1;

		desc->callback = omap_8250_dma_tx_complete;